Recently, as a size of a display device becomes large, a demand of a flat display device such as a liquid crystal display (LCD), a plasma display panel (PDP), and the like increases. These flat display devices have a slow response speed and a limited viewing angle as compared to a cathode ray tube (CRT). Therefore, a study on other display devices for replacing these flat display devices has been progressed.
Among others, a study on an electroluminescence device has been progressed.
As the electroluminescence device in the related art, an inorganic electroluminescence device has mainly been used, but in the case of the inorganic electroluminescence device, it requires a driving voltage of alternating current (AC) 220V or more and should be manufactured in a vacuum state.
As a result, it is difficult to manufacture a large-sized electroluminescence device.
Due to the above-mentioned problems, a study on the organic light-emitting diode (OLED) using organic materials has been progressed. The organic light-emitting diode is a display using self-emission organic materials. When electric field is applied to the organic materials, the organic light-emitting diode (OLED) transfers electrons and holes from a cathode and an anode, respectively, combines them within the organic materials, and uses organic material electroluminescence emitting, as light, energy generated at this very moment.
In the organic light-emitting diode, the phenomenon emitting light may be largely divided into fluorescence and phosphorescence. The fluorescence is a phenomenon that emits light when organic molecules fall from a singlet excitation state to a ground state and the phosphorescence is a phenomenon that emits light when a triplet excitation state to a ground state.
Electrons and holes, which are injected into the organic light-emitting layer configuring the organic light-emitting diode, are recombined to form exciton and light of colors corresponding to energy band gaps of the light-emitting layer is implemented while electric energy of the excition is converted into light energy. In this process, the singlet exciton where spin is 0 and the triplet excition where spin is 1 are generated at a ration of 1:3.
Therefore, when the organic light-emitting diode is generally doped with a fluorescence pigment, maximum internal quantum efficiency is limited to 25%. However, if a spin-orbital coupling is large, the singlet form and the triplet form are mixed, such that an inter-system crossing can be generated between the singlet state and the triplet state.
As a result, the triplet excition can be transitioned while emitting phosphorescence at the ground state. AS a result, all the triplet exciton can be used for emitting light and the quantum efficiency inside the organic light-emitting diode can be theoretically improved up to 100%.
The electrophophorescence diode, which can significantly improve the light-emitting efficiency of the organic light-emitting diode was developed by professor S. R.
Forrest of Princeton University and professor M. E. Thompson's team in the United States (1999). It has been known that complex compounds of heavy atoms such as platinum (Pt), iridium (Ir), europium (Eu), Terbium (Tb), and the like indicate high phosphorescence efficiency. In the case of the platinum complex, the lowest triplet exciton is ligand-centered exciton (LC exciton), but in the case of the iridium complex, the triplet exciton having the lowest energy is metal-ligand charge transfer (MLCT). Therefore, the iridium complex forms the larger spin-orbital coupling to indicate high phosphorescence efficiency while having a much shorter triplet exciton lifespan, as compared to the platinum complex.
An electrophosphorescence diode emitting green light and red light was developed, but a blue electrophosphorescence diode having the excellent light-emitting efficiency, color coordinate, and lifespan is still not developed. As one example, a material called Flrpic (iridium (III) bis [2,2′ (4′-difluorophenyl)-pyridinato-N,C2′] picolinate) has been developed, but cannot considered to be a perfect blue light-emitting material in terms of color purity.